11mm dado over 5m. 40 times.

[ChaiLatte]

I did not see anyone mention it in this form, but you have two options for making the cut:

1) the wood stays still and the tool moves over the wood (e.g. router with guide fence)
2) the tool stays still and the wood moves over the tool (e.g. router table)

To perform the first operation, you need just over five linear metres of space.

In the second, you would need some good infeed and outfeed tables or rollers. You also need ten linear metres of space to do the work.

For a bought-in product, search 'timber i beam'

https://www.jtatkinson.co.uk/i-beams-i-joistshttps://www.thomasarmstrongtimber.co.uk/engineered-timber-products/floors
A one-pass solution would be a challenge. Finding an 11mm diameter router bit would be difficult as it is an odd size. You'd also have to confirm that the groove made by that bit fitted the wood that is to be inserted. Hence, the two pass method suggested above would circumvent that difficulty as it allows you to tune everything to the specific timber you are using.

 

[Alistairmckinnon1961]

PS, the longest length I’ve successfully cut is about 3m. 5m to my mind is a big ask - I think I’d just buy the I beams ready made!

 

[paulrbarnard]

If you look for a ‘bought in’ solution some timber merchants can do profiling for timber purchased. I know Timber Source near me do it.

 

[imageel]

As others have mentioned, there are lots of ways you could do this however to my mind this is better suited to perform on a spindle moulder albeit you need a large workshop to cope with 5m lengths of in/out feed.
Would it be worth reaching out to a few men's-sheds to see if any has one that you could make use of and that has the required space?

 

[Samkilby]

I suppose it all depends really on how accurate the width and location of the slot needs to be. And what timber you are using for the bearers, and how accurate and true it is..
if it’s 5m stuff from the merchants - a router table might actually be worse. If the timber is a big banana, you might never get it against the fence at that cross section it won’t likely bend in.
Are the end pieces needed at 5m, or is this just convenient for budget ? Might be worth chopping to length beforehand if the timber is wobbly, more router passes but might be more accurate..
I think double fence on the big hand held, or it’s fence and then make something for the second one, just so it’s not wobbly. Or make a little sled, mdf and mitrebond then add screws when it’s fit. Use the routers plastic baseplate as a template for drilling it to attach to the sled, attach to not of mdf, plunge, then place it accurately on the timber to be grooved, then attach the 90 degree guides to hold and fit.
If the timber varies in size, use masking tape/bits of veneer/packers to pack the jig out. Or you could make the “fences”adjustable.
I would want the osb in hand so I could measure a selection of the batch with verniers, before ordering a router bit that would do it in one chomp for the width. I would also likely do a test by other means (tracksaw and chisel) before ordering the bit.. Use the multiple depth stop post on the router so you can take multiple height passes, with the bottom one being accurate to depth ?
I wouldn’t take two passes forward and backwards to final width, as the groove size is then dictated by the accuracy of the timber machining..

 

[ChaiLatte]

I wouldn’t take two passes forward and backwards to final width, as the groove size is then dictated by the accuracy of the timber machining..

It would work if the 'groove-width-adjuster' was stuck onto the router's fence. Remove it for the second pass so the cutter moves over and use the same timber edge as the first pass.

Make the first cut in the second timber with it removed and then reinstall for the second cut in the second timber and the first cut in the third timber.

I did wonder with the double fence how it would cope if the timber width varies a bit. Is there such a thing as a spring-loaded fence? Maybe the second fence would have to be like a featherboard, but then that presents difficulties as a feather board is uni-directional, so you'd have to walk back to the beginning to start each cut.

 

[mr rusty]

I'm intrigued why you are making I-Joists when they are readily available off the shelf

 

[MikeJhn]

I don't think a timber size of 47 x 75 qualifies as a joist?

I would suggest a long flat bed of some sort, hot glue the piece to it and then screw in a fence to run the flat edge of the router along it.

 

[Jester129]

10mm slot bit in the router, then dimension to size on a table saw using the carefully set up fence for position?

 

[Adam W.]

I definitely do not have such a long shiny thing, and my wallet shudders at how much I'd have to fork out to make one!

Will look at what the fence situation is for the big router. Would the little router be up to it do you think?

The OF1010 are decent routers and I've got a 1/2" collet on mine, so why not if you take a couple of passes.

Hopefully Festool will make a battery operated one soon and I can ditch the cable......Not that I do much routing these days though..

Anyway as we're British, it's a TRENCH, not a dado.

 

[paulrbarnard]

It would work if the 'groove-width-adjuster' was stuck onto the router's fence. Remove it for the second pass so the cutter moves over and use the same timber edge as the first pass.

Make the first cut in the second timber with it removed and then reinstall for the second cut in the second timber and the first cut in the third timber.

I did wonder with the double fence how it would cope if the timber width varies a bit. Is there such a thing as a spring-loaded fence? Maybe the second fence would have to be like a featherboard, but then that presents difficulties as a feather board is uni-directional, so you'd have to walk back to the beginning to start each cut.

Re double fence: perhaps two pins on opposite sides of the router base. Twisting the router to get one in contact with each side of the material would ensure it is centered. Starting and stopping at the ends would need either starting a little way in with a plunge cut and then sawing off the end or some way of attaching an extra bit to each end.

 

[Sideways]

Since I have two side fences for my routers (just buy 2 of the same router and a pair of long fence rods ...) I'd probably do it that way.
If I was unable to do that, I'd improvise a router table for the job in the form of an 8 foot plank giving me plenty of infeed and outfeed support and no unnecessary width. Just hang the router under the plank.
I'd clamp long timber battens each side as fences to stop the stock moving either way (they would also help the plank resist sagging) and set the whole thing up on trestles outdoors so I had room to feed 5 metres in and out.

 

[hlvd]

Oh.

Up cut spiral bit in the big router with a fence on it or on the track if you have such a long shiny thing. Or your track saw, but that will take time.

I’ve never used a spiral cutter, are the benefits speed of cut due to quicker chip clearance or is it also a cleaner cut?

 

[XTiffy]

I have a solid cast iron router table and there is no way I would cut a groove like that on a router table. I would use a long length of mdf (skirting available in 5.4 metre length), well supported. As you are covering up the end result I would screw each piece down while routing and as has been suggested use a double fence on a large router. The other possible is a hand held cirular saw with a fence cutting a series of cuts from each side in turn.

 

[Adam W.]

I’ve never used a spiral cutter, are the benefits speed of cut due to quicker chip clearance or is it also a cleaner cut?

The up cut ones pull the waste out of the trench, but leave a slightly ragged edge. The down cut ones do the opposite and shove the chips into the trench, which I think is a drag but they leave a clean edge though.

 

[colinb4987]

I don't think a timber size of 47 x 75 qualifies as a joist?

It definitely doesn't...but I-joist is a more recognisable shape/format than if I said I-column, which is closer to how it will be employed. Probably "vertical I-shaped spacer" is most accurate (See Insulation and Cladding thread)

 

[colinb4987]

If I was unable to do that, I'd improvise a router table for the job in the form of an 8 foot plank giving me plenty of infeed and outfeed support and no unnecessary width. Just hang the router under the plank.
I'd clamp long timber battens each side as fences to stop the stock moving either way (they would also help the plank resist sagging) and set the whole thing up on trestles outdoors so I had room to feed 5 metres in and out.

Given I have a 7ft long workbench, a variation of this might well be the way I go.

Any reason to hang the router underneath rather than mount overhead, bridging between the two batten-fences (whether directly mounted or on a plywood scrap, depending on dimensions etc etc)?

 

[colinb4987]

The OF1010 are decent routers and I've got a 1/2" collet on mine, so why not if you take a couple of passes.

Hopefully Festool will make a battery operated one soon and I can ditch the cable......Not that I do much routing these days though..

Anyway as we're British, it's a TRENCH, not a dado.

Good to know, will look into collets.

Battery router - makes sense, though I confess I concluded that for any tool I'm tied to a dust extractor, I might as well also be tied to a power cable.

You finally pushed me to look up dado/groove/trench; I had no idea there were different names for with and cross grain versions. Wikipedia (so mountain of salt required) at least says that:
Cross-grain is a housing (UK), trench (EU), or dado (US)
With grain is seemingly a groove regardless of geography

 

[worn thumbs]

I agree with the double fence approach and I would add a lead in relief to the fences so that by the time the cutter reaches the job,it is positively centred.a router table for a 5 metre length would be rather long by the time you added good support both sides.A couple of passes with a 1/2" router would be my estimate and the mess will be considerable.

 

[Sideways]

Given I have a 7ft long workbench, a variation of this might well be the way I go.

Any reason to hang the router underneath rather than mount overhead, bridging between the two batten-fences (whether directly mounted or on a plywood scrap, depending on dimensions etc etc)?

Not particularly.
The context is that i'm not a fan of router tables unless you are moulding small furniture components. I prefer to take a 1/4" router to the work and not try to manhandle a large workpiece over a silly small table.
But it's totally legitimate to improvise an under or overhead table to suit a specific task.
DIY jigs are what make a router such a great tool.
Your task needs a long narrow one if you can't do the two fence + long rods method.
Anything goes, just make sure you have the router tightly clamped and a big hole around the bit for the chips to clear.
Also, I watch many people newish to the router trying to take a single full depth cut ×hen they should be taking 3 or 5 passes. This is very relevant to slots and trenches because the chips don't clear so easily. To go single pass, the advice above about spiral bits for better clearing is valuable. It may be worth clamping the nozzle of a shop vac close to the cutter in whatever you rig up.

I needed to mould several metres of oval rod and my improv table was an alloy box beam from an old storefront, ally plate vertically up from the box and the router clamped horizontally coming in through the plate. This gave the best support to stop the wood flexing.
You adapt what you have